Flat-panel displays, such as PC monitors, have supplanted CRT (Cathode Ray Tube) displays. One thing CRTs still do better, though, is motion clarity. To address that, some flat-panel displays use a technique known as Black Frame Insertion.
Sample and Hold vs. Pulsed Displays
Why do flat-panel displays need black frame insertion? They have a tendency to exhibit a unique kind of unwanted motion blur that’s caused by how they display and change the image on the screen. An image is held perfectly for the entire duration of a frame across multiple screen refreshes, then it’s almost instantly replaced with the next frame when all the pixels change their state in unison. This is known as image persistence or “sample and hold”.
This sounds like a good thing, but thanks to the way our eyes work, it actually introduces unpleasant motion blur. When your eyes attempt to track an object as it moves across the screen, they are locked onto a different part of the image at the start and at the end of the refresh.
This causes eye-tracking motion blur as the tracking motion of your eye movement causes the image to blur across the retina. There’s more to the reason this causes perceived blur, but the important fact is that it’s exhibited in all flat-panel displays that perfectly show each frame until the next frame is ready. It happens regardless of how fast their pixels can change state.
CRT displays exhibit much less motion blur because with every refresh cycle the entire pixel has to be redrawn or it will fade away. The electron beam draws the images from top to bottom by exciting the phosphor layer on the back of the screen. By the time it reaches the bottom of the image, the phosphors at the top of the screen have already started fading. The period between drawing the last line of the previous refresh and the first of the next is known as the vertical blanking period, where the entire screen is briefly blank. This natural “pulsed” lack of image persistence has a dramatic positive effect on motion blur and is something flat panel makers have tried to emulate in various ways.
Blur Reduction Methods
There are a number of ways in which flat panels attempt to defeat sample and hold motion blur. The one most people are probably familiar with is motion interpolation, also known as motion smoothing. You may also know it as the “soap opera effect“, which is a rather derogatory name for it.
Different TV brands also have their own names for this method, but it all works more or less the same. Let’s say you have content playing at 30 frames per second, but the screen can refresh at 60 frames per second. The motion interpolation generates intermediate frames, which are a sort of average of the frame before and after it. This doubles the number of unique frames and reduces the amount of motion blur. Unfortunately, this generates that silky-smooth motion that makes everything look like GoPro footage or, as we’ve mentioned, soap opera footage.
Some displays are “low persistence” panels. These panels rapidly flicker their pixels (also known as strobing) in a way that mimics pulsed screens. Plasma TVs did this as a natural part of how they work and are inherently low-persistence, despite still being a sample and hold screen technology.
Then we have Black Frame Insertion. Rather than a brief flicker, this method inserts a completely black frame between every lit frame. So the backlight (or the actual pixels in the case of OLED) goes completely dark after every full refresh. This does a good job of mimicking the blanking period of CRTs.
The Benefits of Black Frame Insertion
Black Frame Insertion (BFI) does a great job of defeating eye-tracking motion blur. It tricks your brain into perceiving smooth motion instead. The beauty of this blur reduction method is that you don’t have to mess with the frame rate of the original source footage. Whether it’s 24 frames per second or 60 frames per second, only real unaltered frames are shown, and there’s no soap opera effect.
BFI also works well for applications such as video games. So-called “post-processing” motion blur reduction methods create latency between when the display receives frames and when it displays them. For example, with motion interpolation, the display needs to know what the next frame looks like before it can calculate an interpolated frame to insert between it and the previous frame.
With too much post-processing the time between pulling the trigger in your game of Halo and seeing the gunfire on-screen becomes longer. With BFI you don’t need to do post-processing, since you know every second frame is a black one.
The Downsides of BFI
There are two main drawbacks to BFI. The first is that if every second frame is black with the backlight completely switched off, you are effectively reducing brightness by half. Some modern TVs have a BFI implementation that compensates for this and has a brightness reduction lower than 50%, but you’ll always have a dimmer image with BFI on than with it off. Of course, many modern TVs are so bright that even with BFI on you’ll be happy with the picture. Otherwise, you may have to darken the room to get the best results.
The second potential problem with BFI is that some implementations produce visible flicker. In other cases, some people seem to be better able to see this flicker than others. As you can imagine, flickering images are a recipe for a headache, so BFI that flickers visibly isn’t ideal.
Should You Use BFI?
While there’s no doubt that BFI produces superior motion clarity, the best thing you can do if your display supports it is to switch it on to see if those improvements are worth the tradeoffs. You also don’t have to use BFI for everything. You may want to turn it on for video games or sports, but turn it off for movies, or vice versa. There’s a lot of personal preference at play here but in general, any type of content where motion clarity matters can benefit from BFI.
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